Those are some interesting points about socialization. I know from personal experience that there are unique dynamics in small schools - particuarly private K-12 schools.
Often there are a number of students who have literally gone to school with each other for 12 years. Even if they aren’t friends, they recognize each other’s parents and siblings. When that happens, those boys and girls rarely become attracted to each other in high school. I attribute it to the excessive familiarity.
In any case, I’d say high school dating has little to do with mate selection. Most married couples I know met in college or through their employment.
The “excessive familiarity” is likely something like a psychological-biological effect, that “you closely associate with these children, so they are family, and you shouldn’t marry them.”
I also like to mention that there is a big difference between this kind of socialization and what socialists call socialization. This is strictly so that children learn about each other and are comfortable in social situations. It has nothing at all to do with being “good members of the state”, and “cooperative group activities” being better than individual initiative and effort, or “competition bad, cooperation good.”